chore(deps): add the Dependabot config and drop an unused package

[veelenga]

Overview

• add .github/dependabot.yml: weekly npm and GitHub Actions updates, grouped so three, @types/three and three-mesh-bvh move together, other minor and patch updates come as one runtime and one tooling pull request, and all action updates come as one
• wait 7 days before proposing a new version, and ignore major updates of @types/node so they stay in line with Node 24 from .nvmrc
• drop the globals package: the lint rules that read global names (no-undef, no-redeclare) are switched off for TypeScript files, so it had no effect

Notes

Lint, the full test suite (3,592 tests) and the build pass.

Docker is left out of the config: node:24-alpine should only move together with .nvmrc, and nginx:alpine-slim has no version to update.